Intrusion detector responsive to change in dominant frequency

ABSTRACT

An intrusion detector system using a geophone to sense both ambient or environmental events, as well as man-made or cultural events, and the system being responsive to a reduced number of zero-axis crossings of the geophone output resulting from the superimposing of man-made events upon ambient events to detect intrusions. Zero-axis crossings are represented in a long-term memory and in a short-term memory or accumulator, and their contents are compared to determine when short-term conditions change as compared with averaged long-term conditions representing mainly environmental events. In one embodiment, when an intrusion is detected in this way the system requires at least one more such detection before setting off an alarm. The memory is updated with more recent information, but only when conditions appear to be ambient, and relatively free of unusual events.

United States Patent 3,686,658 1 Aug. 22, 1972 Wilt [54] INTRUSION DETECTOR RESPONSIVE TO CHANGE IN DOMINANT FREQUENCY [72] Inventor: Marvin D. Wilt, Dallas, Tex.

[73] Assignee: Teledyne Industries, Inc., Geotech Division [22] Filed: May 12, 1970 [21] Appl. No.: 36,603

[52] [1.8. Cl. ..340/261, 340/258 D, 235/92 CA [51] Int. Cl, ..G08b 13/00 [58] Field of Search ..340/261, 258, 258 A, 167, 258

D, 340/168; 235/92 DP, 151.3, 92 CA [56] References Cited UNITED STATES PATENTS 3,111,657 11/1963 Bagno ..340/258 A 3,331,065 7/1967 McDonald ..340/258 A 2,730,896 1/1956 De Boisblanc ..340/261 UX 2,903,683 9/1959 Bagno ..340/261 X 3,465,339 9/ 1969 Mamer ..340/261 UX 3,344,277 9/1967 Smith et al ..340/261 UX FOREIGN PATENTS OR APPLICATIONS 942,848 11/1963 Great Britain ..340/261 UX Primary Examiner-John W. Caldwell Assistant Examiner-Michael Slobasky Attorney-Alexander and Dowell ABSTRACT An intrusion detector system using a geophone to sense both ambient or environmental events, as well as man-made or cultural events, and' the system being, responsive to a reduced number of zero-axis crossings of the geophone output resulting from the superimpos- 15 Claims, 4 Drawing Figures ALARM I \l a I24 VOLTAGE RATIO 9 THRESH o COMPARISON CIRCUIT lOSec. INTEGRATOR 4, H3 III LONG- snow- TERM TERM p/IZZ MEMORY MEMORYJ I26 llO I25 v I07 I22 SOUARING FREQ. VOLT. J A AMPLIFIER DISCRIMINATOR -Ioe AGC f I04 I04 IoI INTRUSION DETECTOR RESPONSIVE TO CHANGE IN DOMINANT FREQUENCY This invention relates to intrusion detector systems, and more particularly to systems using either digital or analog techniques to process noise information and determine therefrom the existence of an intrusion.

It is a major object of this invention to provide an intrusion detection system capable of unattended automatic operation and capable of maintaining under surveillance a relatively large area.

Another major object of the invention is to provide a system having a lower false-alarm rate than most prior art systems.

Another object is to provide a system using a geophone as the detection device, which geophone is rugged and dependable and provides omni-directional information.

It has been recognized in the past that usually natural or environmental events have a voltage characteristic which is random in distribution, whereas man-made or cultural events tend to be periodic in nature. For instance, U.S. Pat. No. 3,331,065 to McDonald teaches this difference and uses it in the performance of its surveillance. The present system recognizes further differences between such events and uses the fact that during ambient noise conditions the frequency and therefore the rate of zero-axis crossings will be relatively higher than the frequency of man-made events of the type to which the present system is especially sensitive, for example, the tramping of feet on the ground, or the rumble of a vehicle thereover. When the latter type of intrusion occurs, the lower frequency component of the man-made noise will be superimposed upon the higher-frequency ambient noise components and will thus displace the latter away from the zero-axis except in the vicinity of the formers zero-axis crossings. The result will be that the number of zero-axis crossings per unit time of the composite components will be greatly reduced.

The digital form of the present system operates by counting pulses into a counter over a long-term interval of time, and then setting that number into a memory provided no alarm and no signal drop-out occurs during that interval, and the number is made to vary inversely as the frequency of zero-axis crossings of the geophone signal. By making the interval of counting rather long, conditions in the surveillance area are averaged out. On the other hand, during relatively short intervals of time the system counts pulses into an accumulator, but at a rate which is made higher proportionally as the interval is made shorter. Again, the actual duration of this interval determines the number of pulses entered into the accumulator, i.e. the number is determined by the rate of occurrence of the most recent zero-axis crossings. These relatively slower and faster pulse counting rates used to feed the memory counter and the accumulator are differently scaled to take account of the difference in the intervals during which each counting device receives pulses. The accumulator count must reach the memory count before the accumulator is reset in order for the system to detect an intrusion. The zero-axis crossings are converted to two different trains of impulses which are mutually scaled to provide reset pulses to the accumulator more frequently than reset pulses are applied to the memory counter. Thus, assuming that the memory contains pulses counted at a slower rate but over a longer period of time, these pulses can represent average ambient conditions. Now, if the accumulator counts pulses at a higher rate but over only a short interval, because it is reset quite often as compared with the memory counter, it will be able to reach coincidence with the number of counts in the memory when an intrusion occurs because the zero-axis crossings will become less frequent on a short-term basis and therefore the accumulator will have been permitted to count longer. Meanwhile, the number in the long-tenn memory remains temporarily fixed. Thus, a temporary reduction in the number of zero-axis crossings attributable to an intrusion will permit the accumulator to count more pulses and easily reach and surpass the number in the memory and thereby detect an intrusion.

It is an object of this invention to provide a digital system in which clock pulses count long-term and short-term memory counters upwardly, and the output of the geophone is used to reset both counters after having scaled the number of zero-axis crossings of the geophone-output according to the differences in longterm and short-term clock pulse rates entered into the long-term and short-term pulse counters. By comparing the short-term count with the long-term count, recent changes in the former occurring before the memory of the latter is changed can be digitally detected and used to indicate an intrusion. v

Another object of the invention is to provide a system in which the output of the geophone is squared up so that the zero-axis crossings can be detected, and the input amplifier is provided with automatic gain control in order to give it a large dynamic range. This squaring amplifier clips the wave forms of the geophone output and drives two dividers which scale the zero-axis crossing impulses down so as to provide two trains of impulses, one of which occurs relatively less frequently and is used to reset the memory counter, and the other of which occurs relatively more frequently and is used to reset the short-term pulse accumulator, which has had pulses counted into it at a proportionately higher rate.

Another object of the invention is to provide a system having two different criteria imposed upon the short-term pulse accumulator such that the first detection of coincidence with the count in the long-term memory is relatively more difficult to arrive at than the second detection of coincidence using different parameters. The system thus requires successive detections of coincidence before an alarm signal can be delivered.

Still another object of the invention is to provide a system having alarm means and having intrusion detection means, said system including a divider which must be actuated a predetermined number of times by intrusion detection signals before the alarm means will be actuated.

A further object of the invention is to provide a system in which the intrusion signals required to set off an alarm must comprise an unbroken succession before the alarm will be actuated.

Another important object of the invention is to provide a system in which the memory which remembers the digital number representative of ambient background conditions is updated by a counter which is counted upwardly over long-term intervals whose duration is determined by the frequency of zero-axis crossings of the output from the geophone. The new number in the counter can be used to update the number in the memory, or it can be dumped. The new number is used to update the memory if it appears to represent ambient conditions, but it is dumped if during the interval of counting of such number into the counter the system detects an alarm, meaning that the count is not representative of substantially ambient conditions. Moreover, if all background noise fails completely so that the AGC circuit of the input amplifier indicates no noise level, the number in the counter will be dumped because this number may be meaningless in view of the fact that there has been no steady train of reset signals available to reset the counter and therefore the counter may have been simply counting amplifier input noise for an indeterminate period of time and its content may be meaningless. As long as the count in the counter is not transferred to the memory, the memory retains the last digital number set into it from the counter and used for comparison in the comparator with successive numbers which are being counted by the accumulator to represent conditions determined on a short-term basis.

Another object of the invention is to provide an analog embodiment in which the zero-axis crossings are counted and converted into a voltage level proportional thereto. Two memories are used to average this level, one over a long-term such as 30 minutes, and the other over a short-term, such as 3 seconds. While the number of zero-axis crossings remains substantially constant, both will average the level to about the same output voltage, but when an intrusion occurs and the number of zero-axis crossings falls off, the short-term memory voltage level will also fall off rather rapidly and the system will alarm when the short-term level reaches, for example, 70 percent of the long-term level. This version of the system is also provided with safeguards to prevent the long-term voltage level from being changed under abnormal conditions, i.e. during input signal dropout or during alarm conditions.

The range of frequencies to which the present system is sensitive includes approximately to 100 Hz. Wind noise may provide a signal at about 80 or 90 Hz, whereas vibrations caused by man-made noise frequently lie in the vicinity of or 30 Hz. Although a number of different types of transducers can be used, for example, microphones, one advantage of the use of a geophone is that its best frequency response lies approximately within the above stated spectrum. Moreover, tests have shown that the geophone has a reasonable distance range, such that several persons walking in an open field can be detected by the system at several hundred yards range, and the rumble of a vehicle will cause detection at a considerably greater distance. One characteristic of man-made noise is that it usually occurs for a considerable period of time, for instance, in the case of persons walking or marching or in the case of a vehicle or vehicles passing the geophone. On the other hand, isolated noises such as a clap of thunder, or something falling on the ground from a tree will produce in the memory counter only a very small change in the number of counts so that,

averaged over a period of time, the event may be considered insignificant. Moreover, the occurrence of an isolated event will not be of sufficient duration to set off an alarm because of the fact that it will not produce the predetermined number of successive intrusion signals required by the system before an alarm is actuated. It may cause one or two intrusion signals, but it should be recalled that the present system does not alarm until there have been a number of consecutive intrusion signals occurring in unbroken sequence. Thus, the man-made noise is more apt to provide a continuous sequence than isolated claps of thunder, objects falling out of trees, etc. There are certain man made noises which can be ignored by the system as being more in the nature of ambient environmental conditions. For instance, the noise from aircraft is of somewhat higher pitch than marching or vehicle rumbling noises, and would therefore probably not tend to reduce the number of zero-axis crossings sufficiently to actuate an alarm even though the noise may be present at a relatively high amplitude. The present system is substantially insensitive to amplitude provided the intensity of the input to the geophone is within the AGC capability of the amplifier.

Other objects and advantages of the invention will become apparent during the following discussion of the drawing, wherein:

FIG. 1 is a block diagram showing a digital intrusion detector system according to the present invention;

FIGS. 2 and 3 are related wave shaped diagrams showing the decrease in zero-axis crossings resulting from the superimposing of lower frequency man-made components upon higher frequency ambient noise components; and

FIG. 4 is a block diagram showing an analog intrusion detector system according to the invention.

Referring now to the drawings which show illustrative embodiments of the present invention, these embodiments use memory techniques to distinguish between events which are of a natural environmental character and can therefore be considered to be ambient events, and non-ambient events of a cultural or man-made character. As stated in the earlier part of this specification tests have indicated that the character of ambient events which can be picked up by a vibration transducer is rather different from the character of man-made events picked up by the same instrument. The frequency of the ambient events tends to be higher, perhaps in the region of 50 to Hz, whereas the frequency of man-made events, such as persons walking in the vicinity or such as vehicles rumbling over a highway or a field, tends to be lower, perhaps as low as 10 Hz or more. The frequencies mentioned above are or course the lowest frequency content of the signals whose higher frequency content is apt to be smaller in amplitude and can generally be ignored for present purposes. FIG. 2 shows a typical ambient condition in which the higher frequency wave form H has a relatively larger number of zero-axis crossings. On the other hand, FIG. 3 shows the same higher frequency ambient components in the presence of lower frequency components L which of course cross the zero-axis 0 rather less frequently than the higher frequency components H shown in FIG. 2. The fact that the lower frequency components L displace the higher frequency components toward both sides of the zero-axis tends to drastically reduce the number of axis-crossings. These components are picked up by the geophone and comprise the input of the system about to be described.

DIGITAL DETECTOR SYSTEM Referring now to the diagram of FIG. 1, this system illustrates a typical embodiment of the present invention which includes a long-term counter and a shortterm accumulator memory 20 both of which count pulses from a common oscillator 22, but at different rates and for different lengths of time. The counter 10 is arranged to be reset fairly infrequently and to count pulses delivered at a certain rate to reach a total number of pulses counted in over the long-term period of time, during which interval averaging is to occur. The memory level in the counter 10 is therefore intended to represent ambient background conditions, arrived at substantially in the absence of man-made events. On the other hand, the accumulator memory 20 counts pulses for a very much shorter period of time, but the pulses are introduced into it at a higher rate than those entered into the counter 10, and therefore the accumulator 20 is seeking to provide a memory level which is representative of recent events occurring during a relatively short interval of time. The philosophy behind the use of the counter 10 and the accumulator 20 is that when man-made events begin to occur they will have a big effect upon the count of the accumulator 20 which receives pulses at a high rate but for a brief period of time, whereas these same man-made events will have a relatively small effect upon the count in the counter 10, which counts pulses slowly but over a relatively greater interval of time. Under these conditions if count levels from the accumulator memory 20 and from the counter 10 are compared, the accumulator 20 will have totalled more pulses during its short length of time and therefore its count will have passed coincidence with the count from the counter 10 as a result of the intrusion of man-made noise. The comparison is not actually made of the count in the accumulator 20 with respect to the count in the counter 10, but with respect to the count in a memory register 12 into which a prior count of the counter 10 has been set, as will be described below.

A comparator serves to make this comparison and, if its criterion of coincidence is met, to deliver an intrusion signal on the wire 16, which seeks to set 0E an alarm in a manner to be hereinafter explained. The level in the counter 10 under certain circumstances will be representative of ambient conditions, but under other circumstances may be non-representative. Therefore, its total count is not always set into the memory 12, which is actually connected to the comparator 15 instead of the counter 10. The AND gates 14 serve to control whether or not the count in the counter 10 is to be set into the memory 12 for comparison in the comparator 15, or whether a previous count set in the memory 12 is left therein and the present count in the counter 10 is merely dumped when the counter is reset, in the manner to be hereinafter explained.

As indicated above, the oscillator 22 supplies pulses both to the counter 10 and to the accumulator through a divide 24, and the rate of these pulses in this embodiment is 125 Hz, which is an arbitrarily selected pulse rate. This pulse rate is normally applied through Both the counter 10 and the accumulator 20 must be reset from time to time, and the circuitry in the lower part of the diagram of FIG. 1 serves this purpose. A vibration transducer, in this case the geophone 40, includes a probe 41 which is in contact with the ground G and receives the noise waves H and L, FIGS. 2 and 3. The output of the geophone 40 is amplified in an amplifier 42 and the gain of this amplifier is controlled by an AGC circuit 44 comprising a loop coupled around the amplifier to extend its useful range. A squaring amplifier 43 saturates, and therefore its output is a square wave having zero-axis crossings. This square. wave is used to drive a 64 circuit 46 delivering an output on the wire 47. Assuming that the number of zero-axis crossing pairs is somewhere between 10 and per second, this means that the output on wire 47 will be an impulse occurring somewhere between once every several seconds, and several times per second. It would be a reasonable assumption to consider that the impulse will occur on an average basis about once per second. The occurrence of this impulse is again divided by another scaler 50 by a factor 128:1, and therefore if the impulse on wire 47 were assumed to occur roughly once per second on an average basis, then the impulse appearing on wire 51 would occur about once every 2 minutes, more or less. The pulse on wire 47 is used to reset the accumulator 20 and the pulse on wire 51, after passing through a brief delay circuit 52 serving a purpose to be hereinafter explained, is applied on wire 53 to reset the counter 10. Thus, it can be seen that neither the counter 10 nor the accumulator 20 is reset at a regular rate, but instead both are reset at a rate depending upon the number of zero-axis crossings occurring during any particular interval. The fewer the zero? axis crossings, the longer the counter 10 and the accumulator 20 are permitted to count pulses in view of the fact that the accumulator is reset after every 64th zeroaxis crossing pair and the counter 10 is reset less often by a factor of 1:128.

Recalling from what has been stated previously that the counter 10 is intended to slowly receive pulses over a longer counting interval, namely between impulse reset signals on wire 51, perhaps averaging in the vicinity of 2 minutes apart, when the counter reaches its highest count just before being reset by an impulse signal on wire 53, a determination is made as to whether or not the count in the counter 10 is reasonable representative of ambient noise conditions and should therefore be transferred to the memory circuit 12 for comparison in the comparator 15. Obviously, if an alarm has occurred during the counting interval between pulses on wire 53, it cannot be said that the count in the counter 10 is representative substantially only of ambient conditions. Likewise, if the signal from the geophone 40 has disappeared altogether for any part of that counting interval, meaning complete lack of noise events, the counter may not have a representative count at all, because it may have counted pulses which are the result of amplifier input noise. In this event, the count in the counter 10 might be incorrect and this might result in a false alarm occurring as soon as ambient events begin occurring again, even though they may not be representative of an intrusion. Therefore, the AGC circuit 44 is used to deliver an output on wire 45 to an inverter circuit 48 such that when the AGC indicates the presence of signals in the amplifier 42 there will be no output on the wire 49, but when the signals virtually fail in the amplifier 42 the wire 49 will be energized. Thus, the occurrence of an alarm or the occurrence of a signal failure, either one, will be signalled through the OR gate 54 to set a flip-flop 56 and remove an enabling signal from the wire 57. The result is that the pulse appearing just before reset of the counter 10 on the wire 51 will be blocked from passing through the AND gate 58 to set the count of the counter 10 through the AND gates 14 to update the memory 12 because the wire 59 will not be enabled. Therefore, the memory will continue to retain a previously remembered ambient condition count rather than be updated by the more recent count in the counter 10 whenever an alarm has been sounded during the interval of time when the counter 10 is being counted upwardly or when a loss of noise events is signalled by the AGC circuit 44 through the wire 49. A slight delay is introduced into the wire 53 by the delay circuit 52 so as to give the gates 14 time in which to set the count of the counter 10 into the memory 12, provided the wire 59 is energized, before the count in the counter 10 is reset to zero by the delayed impulse on the wire 53, which impulse also resets the flipflop 56 thereby restoring the enabling signal on the wire 57. During the next succeeding count of the counter 10 if no alarm signal is passed through the OR gate 54 and no loss of events occurs at the input of the geophone 40 as signalled by the wire 49, the next time an impulse appears on wire 51, it will pass through the AND gate 58 and enable the wire 59 which in turn enable the gates 14 to set the count of the counter 10 into the memory 12 just before the delayed pulse on the wire 53 resets the counter 10 to zero to begin a new counting interval. In this way, each number reached by the counter 10 is tested prior to reset to determine whether it is reasonably representative of ambient conditions, and if it is, it is used to update the memory 12 with a new digital number. The less frequent the zero-axis crossings, the higher the numbers reached by the counter 10 and by the accumulator 20.

The comparator can be a series of gates ANDed together to determine when the count in the accumulator just reaches the count in the memory 12, at which time a coincidence signal output appears on the wire 16. This output signal occurs if the accumulator 20 is able to accumulate a count of pulses equal to the number of pulses in the memory 12 during the relatively brief interval of time that the accumulator 20 is permitted to count pulses entered into it on the wire 32 through the OR gate 30. Assume for instance that the memory contains a certain number of pulses acquired as an average count and transferred to the memory 12 during a time when no low-frequency events occurred, at least not in significant number. The accumulator 20 is counted upwardly from pulses entering the gate 28 via the wire 25 at one-half the oscillator rate. If the rate of zero-axis crossings remains exactly constant, one would reasonably expect the accumulator 20 to accumulate the same number of pulses before it is reset as the number of pulses which would be counted by the counter 10 before it is reset, in as much as the counter 10 would be reset only 1:128th as often as the accumulator 20 but is counting pulses entered into it at 1:]28th the rate of the pulses which are entered into the accumulator 20. Therefore, it is relatively easy for a first signal to be detected at the output of the comparator 15 as a result of the accumulator count reaching coincidence with the count stored in the memory 12. However, when this first signal is detected, a changeover is made in the counting rate of the accumulator 20. This changeover is a change in a system parameter, namely. the counting rate, by doubling it and taking the pulses from wire 23. Accordingly, if an intrusion occurs and the number of zero-axis crossings suddenly decreases as shown in FIG. 3, the accumulator 20 will begin to be reset far less frequently and therefore its count will go higher before being reset. The new parameter requires that the total number of zero-axis crossings from reset to the second coincidence be 1% times the count in the memory 12.

It has been assumed to date that the multivibrator 62 was initially toggled to a condition in which an output is occurring on the wire 64, thereby enabling the gate 28 but disabling the gate 26 since there is no output on the wire 63. Since the wire 63 is de-energized, the AND gate 66 is also de-energized and therefore the first signal on wire 16 does not pass through it. However, this first signal energizes the wire 16 and toggles the multivibrator 62 to remove the signal from wire 64 and shift it to enable the wire 63. In this way, the gate 26 is enabled and the gate 28 is blocked, and therefore pulses are now counted into the accumulator 20 through the OR gate 30, but at twice the rate at which they were previously counted in directly from the wire 25 via the gate 28. The accumulator 20 now begins counting upwardly again but at twice the rate. As a result, the zeroaxis crossings and the resulting reset impulses applied via the wire 47 must now be occurring at only twothirds of their previous rate in order for the accumulator 20 to accumulate a sufficient number of oscillator pulses so that the number stored in the memory 12 can be reached by the accumulator 20 twice before being reset by the wire 47. If this condition occurs, the comparator will deliver a second coincidence signal on wire 16 putting an output signal on wire 16 into the AND gate 66 which is now enabled because the wire 63 is now enabled. This second signal will advance the 4 scaler 68 by one count. The second coincidence signal appearing on wire 16 does not reset the multivibrator because it can only be reset by a signal on wire 47. Therefore, only one count will be registered normally in scaler 68, since random frequency shifts are not normally sufficient to reach coincidence several times between reset impulses on wire 47. However, when an intrusion does occur so that the frequency of zero-axis crossings drops off and remains reduced for a considerable period of time, the result will be that coincidence signals will occur in sufficient number that eventually the 4 scaler 68 will overflow and the alarm system 7 0 will be set off to warn of the intrusion.

Since the alarm signal wire 69 has been energized, a signal will pass through the OR gate 54 to reset the flipflop 56 and thereby disable the gate 58 so that the content of the counter 10, which has been acquired during an intrusion interval and therefore cannot be said to represent ambient conditions, will be blocked by the disabled gates 14 so that it cannot update the memory 12, which memory will continue to remember ambient conditions occurring prior to the intrusion. Thus, the accumulator 20 in order to advance the scaler 68 must find several pairs of intrusions, one occurring while the accumulator 20 is being counted upwardly at a lower halved rate, and the other occurring while the accumulator is counted at the full rate of the oscillator 22. It is of course to be understood that the present criteria for sounding the alarm are arbitrarily selected, and that other criteria are within the scope of the present disclosure, for example, attainable by changing the number of counts required to overflow the scaler 68.

There is the possibility that the accumulator 20 may fail to reach coincidence when counting at the full oscillator rate, while the wire 63 is enabled and the wire 64 is disabled. If this were to occur, there would be no intrusion signal on wire 16. Therefore, the next reset impulse on wire 47 will return the multivibrator 62 to its initial condition wherein the wire 64 is energized and the wire 63 is not. As a result thereof the entire system commences looking for a new intrusion condition.

There is the further possibility that the accumulator might reach a coincidence count several times in a row, but fail to find an intrusion as many times in a row as are required by the scaler 68. If this should occur, the circuitry 72, 74, and 76 serves to reset the scaler 68 to zero upon failure to detect an intrusion prior to overflow of the scaler 68. It will be recalled that each time the accumulator register 20 is counted upwardly, it is reset by an impulse on the wire 47, and this same impulse can be used to reset the scaler 68 if no intrusion is detected. Whether the impulse on reset wire 47 is permitted to pass through the AND gate 76 to the reset wire 77 depends upon the condition of the flipflop 74. When the accumulator begins counting upwardly, if it should reach the count in the memory 12 before the occurrence of the reset impulse on the wire 47, an output from the gate 66 can set the flipflop 74 and thereby remove the enabling signal from the wire 75 in order to block the gate 76. Therefore, when intrusion is detected the reset impulse is prevented from resetting the scaler 68 via the wire 77. However, a brief delay is imposed by the circuit 72, and after this delay, when the accumulator 20 has just been reset by the impulse on wire 47, the delayed impulse on wire 73 resets the flipflop 74. During the next accumulated count before the impulse on wire 47 resets the accumulator 20, if intrusion is detected by an output on wire 16 the flipflop 74 is again set to block the gate 76 and thereby prevent resetting of the scaler 68.

However, if no intrusion is detected on wire 16, the flipflop 74 will remain in reset condition and will apply an enabling signal on wire 75 to the gate 76. Whenever the enabling signal is present on wire 75, the next reset impulse on wire 47 passes through the gate 76 and resets the scaler 68 via the wire 77. This reset action will of course occur when the scaler 68 is already reset,

meaning that no intrusion signals have been detected for some time, and therefore, the reset on wire 77 is not needed, but on the other hand it causes no harm.

By the above means, it will be seen that the gate 66 actually has to output normally in 4 consecutive time intervals between impulses on wire 47 before the alarm can be set off by an alarm output on wire 69. If the intrusion detections are not consecutive, and a noncoincidence condition occurs within the 4 reset periods of wire 47, the scaler 68 is reset by an impulse on the wire 77 and the system has to commence again looking for a new series of detections. If during ambient noise conditions the pulses on wire 47 appear, for example, approximately every second, they should occur during detections of intrusions at the rate of about once every 2 or 3 or more seconds and therefore it should take approximately 8 or more seconds in order to set off the alarm 70 after an intrusion begins occurring, and this is a very short period of time as compared with the 2 minutes or more, counting time of the counter 10 when detecting ambient background conditions. Therefore, it will rarely occur that the counter 10 will be reset within the alarm detecting sequence of the accumulator 20.

The present invention does not specify a particular type of alarm system 70, but this alarm may-either be attached directly to the circuit shown in FIG. 1, or else it can be a remote alarm connected to the circuit by telephone wires, or by a suitable radio link where the alarm system is intended to be remotely located over a great distance, or where it is not convenient to deploy wiring.

This digital embodiment shows an illustrative system of gates which is capable of considerably alteration and variation, without changing the main features of the invention in which one memory counter averages a level over a long period of time but at a slow rate, so as to average out and ignore isolated events, and in which another accumulator memory is used for rapidly accumulating a count level at a high rate but over a brief period of time so that the accumulator is more sensitive to the occurring of man-made events. The ambient condition counter enters its digital count into the longterm memory only if the count fairly represents ambient conditions, and the numbers thus set into the latter memory are compared with the accumulated short-term memory counts representing events occurring over a much shorter and more recent interval of time, and coincidence is used as a criterion to set off an alarm provided the digital content of the accumulator reaches the digital content of the long-term memory not just once, but a predetermined number of times in succession where that succession is unbroken by failures. Obviously, changes in circuitry may be made in the particular illustrative embodiment without de-' parting from the concept of the invention.

ANALOG DETECTOR SYSTEM FIG. 4 illustrates a second embodiment of the invention showing an analog system for detecting intrusions based upon a reduction in the number of zero-axis crossings of signals from a geophone sensor 100. This analog embodiment resembles the digital embodiment of FIG. 1 to the extent that it also involves a long-term and a short-term memory device, the long-term memory keeping track of ambient conditions as averaged over a long period of time, for instance, 30 minutes, and the short-term memory keeping track of very recent events, occurring during the last 3 seconds for example.

The geophone 100 delivers its signals through the wire 101 into a high gain amplifier 102 whose amplification is controlled by an automatic gain control circuit 103, which takes an output signal from the amplifier 102 and delivers an AGC voltage on wire 104 to regulate the gain of the amplifier 102 and provide an output level which is suitable for feeding into a squaring amplifier 106. The output of the latter comprises a constant amplitude square wave preserving the zero-axis crossings detected by the geophone, over an approximate 9O decibel range of input amplitudes. The output of the squaring amplifier 106 is delivered to a linear discriminator 108 which converts the frequency of the input signals to an output voltage, whose level is linearly proportional to the number of zero-axis crossings of the squared input signal introduced on wire 107.

The output voltage level appearing on wire 109 from the linear discriminator 108 is applied to the two memory circuits 110 and 112. Both of these memory circuits comprise averaging voltage-storage circuits, but with vastly different time constants. The short-term voltage memory 110 has a time constant of approximately 3 seconds, whereas the long-term voltage memory circuit 112 has a time constant of about 30 minutes. Both of them are charged by the voltage appearing on the wire 109, but because of the long time constant of the reference memory 112 its voltage will be only slightly affected by short-duration variations in the voltage appearing on wire 109 which represents the momentary rate of zero-axis crossings. The long-term averaged memory voltage appears on wire 113.

On the other hand, in view of the relatively shorttime constant of the voltage memory 110 its output level on wire 111 will be rather substantially affected by short-term variations of the voltage level on the wire 109. Thus, it is possible to compare the long-term averaged voltage on wire 113 with the short-term averaged voltage on wire 111 using a voltage ratio threshold circuit 115. If ambient noise conditions remain steady, meaning that the number of zero-axis crossings is relatively constant, then both memories 110 and 112 should be storing the same voltage. However, when an intrusion occurs and the number of crossings decreases, the voltage level in the short-term memory 110 therefore drops below the level in the long-term memory 112. The threshold circuit merely comprises a gate which is adjusted to provide an alarm criterion such that, whenever the short-term signal voltage on wire lll drops by a certain percentage below the long-term reference voltage on wire 113, the threshold circuit 115 will provide an output on wire 116. The percentage drop of the voltage on wire 11] below the voltage on wire 113 required to produce an output from the threshold circuit 115 is a selectable value, and a typical example of that value might be of the order of 70 percent. In other words, as soon as the short-term voltage level on wire 111 drops to 70 percent of the long-term reference voltage level on wire 113, an output will appear on wire 116. This output passes a signal through the normally enabled alarm gate which actuates the alarm circuit 1 19.

However, it is not desirable that the alarm be sounded under certain conditions. For instance, if all signal is lost at the geophone, it is not desirable that the alarm be set off even though the voltage on wire 111 may have fallen well below the voltage on wire 113 after a short period of time. Therefore, the AGC level appearing on wire 104 is applied to an inverting amplifier 121 which provides a zero output on wire 122 as long as an adequate level of AGC is maintained on wire 104 indicating the presence of a substantial signal. If the signal disappears, however, the loss of AGC signal on wire 104 causes the inverting amplifier 121 to deliver an inhibit output on wire 122. This output is fed into a lO-second non-linear integrator 123 which has a negligible delay on drop-out, but a lO-second delay on return of signal, and which puts out a signal on wire 124, and this signal operates to inhibit the gate switch 120 and keep it inhibited during signal dropout to thereby prevent an alarm when there is no input signal of substantial level from the geophone 100 into the system. The inhibit signal on wire 124 will also persist for a while after an input signal again resumes at the geophone.

Likewise, it is not desirable to update the voltage level in the reference memory 112 during certain unusual ambient conditions. For instance, if there is insufficient input signal arriving from the geophone, the reference memory should not be charged via the wire 109 through the gate 125 and the wire 126, because the inhibit voltage appearing on wire 122 will inhibit the gate 125. Another circumstance under which it is better to have no change made in the long-term reference voltage level is during an actual alarm condition. Therefore, the voltage on wire 116 is used to also inhibit the gate 125 and prevent changing of the reference memory voltage on wire 113 while the alarm 119 is actually operative.

The embodiment of FIG. 4 is intended only to illustrate one possible way in which analog techniques can be used to up-date long-term and short-term memories in a system wherein the short-term memory is compared with the long-term memory to determine when an intrusion has taken place.

The following claims are made covering features described and illustrated in the preceding specification.

I claim:

1. A system for detecting an intrusion into a surveillance area resulting in a change in the noise characteristic within the area from ambient type noise to acomposite of ambient type noise and cultural type noise, causing a substantial shift in the noise frequency spectrum, said system comprising:

a. transducer means for picking up vibrations caused by ambient and cultural noise events in the area;

b. means coupled to receive output from the transducer means for converting said picked up vibrations into input signals representative of the momentary dominant frequencies of the noise events;

c. long-term memory means coupled to said converting means and operative to store and average over a relatively longer period of time a level representative of the average frequency of said input signals over that period;

d. short-term memory means coupled to said converting means and operative to store and average over a relatively shorter period of time a level representative of the average frequency of the most recent of said input signals;

e. means for indicating an intrusion; f. means connected between said memory means and said indicating means for comparing said longterm and said short-term memory levels, and responsive to a predetermined criterion of comparison and to the results of said comparison to actuate said intrusion indicating means;

AGC amplifier means connecting said transducer means to said converting means and operative to develop an AGC level proportional to the level of the noise events; and

h. means responsive to said AGC level to block actuation of said intrusion indicating means upon substantial failure of the input noise from said transducer means.

2. In a system as set forth in claim 1, said converting means comprising means for converting the noise events into a voltage signal whose amplitude varies as the frequency thereof; and said memory means each comprising an averagingvoltage-storage circuit each connected to receive said voltage signal and respectively having longer and shorter time constants, and said comparing means comprising a voltage comparison circuit connected to said memory means and operative to compare the voltage levels stored therein, and said comparison circuit being operative as long as said AGC level shows no failure of input noise to deliver an actuating signal to said intrusion indicating means when the ratio of said levels changes beyond a preset criterion.

3. In a system as set forth in claim 2, means responsive to the AGC level in said amplifier means and operative upon substantial failure of input noise from the transducer means to block input of said voltage signal into said long-term memory means.

4. In a system as set forth in claim 3, said means for blocking said voltage signal further including means responsive to the delivery of an actuating signal to said intrusion indicating means for blocking said voltage signal to said long-term memory means.

5. A system for detecting an intrusion into a surveillance area resulting in a change in the noise characteristic within the area from ambient type noise to a composite of ambient type noise and cultural type noise, causing a substantial shift in the noise frequency spectrum, said system comprising:

a. transducer means for picking up vibrations caused by ambient and cultural noise events in the area;

b. means coupled to the transducer means for converting said picked up vibrations into input signals representative of their momentary dominant frequencies, said converting means converting the noise events into pulse signals;

. long-term memory means coupled to said converting means and operative to store and average over a relatively longer period of time a level representative of the average frequency of said pulse signals over that period;

d. short-term memory means coupled to said converting means and operative to store and average over a relatively shorter period of time a level representative of the average frequency of the most recent of said pulse signals, said memory means each comprising a counter operative to accumulate a digital count level;

. a source of clock pulses scaled to count pulses at a higher rate into the short-term memory counter and at a relatively lower rate into the long-term memory counter;

f. means responsive to said pulse signals for scaling them according to the same ratio as said lower and higher pulse rates to provide two series of reset pulses, the less frequent reset pulses being connected to reset said long-term memory counter and the more frequent reset pulses being con nected to reset said short-term memory counter;

g. means for indicating an intrusion;

h. means connected between said memory means and said indicating means for comparing said longterm and said short-term memory levels, said comparing means comprising means for detecting coincidence of the digital count levels in the counters and responsive thereto to deliver an actuating signal to said intrusion indicating means.

6. A digital system for detecting an intrusion into a surveillance area resulting in a change in the noise characteristics within the area from ambient type noise to a composite of ambient type noise and cultural type noise causing a substantial shift in the noise frequency spectrum, said system comprising:

a. transducer means for picking up the ambient and cultural noise frequencies; a source of pulses; a long-term counter means, a short-term accumulator means, and pulse rate scaler means operative to supply pulses to the long-term counter means at a first slower rate and pulses to the short-term accumulator at a second faster rate; means for converting the output of said transducer means into impulses representing zero-axis crossings and for scaling said crossing impulses to provide two different trains of reset outputs whose relative recurrence rates are in the same ratio as said faster and slower pulse rates;

e. means for periodically resetting said long-term counter means using the less frequent reset outputs and for resetting said short-term accumulator means using the more frequent reset outputs;

f. means for comparing the counts in said counter means and said accumulator means and for delivering a coincidence signal indicating an intrusion when the counts reach coincidence.

7. In a system as set forth in claim 6, memory means interposed between said long-term counter means and said comparing means, transfer means for setting into the memory means a new count from the counter means just before the latter is reset, and means for inhibiting said transfer means from updating the count in the memory means when an intrusion detection signal has been delivered from the comparing means since the last time the counter means was reset.

8. In a system as set forth in claim 7, amplifier means coupled to the transducer means, AGC means in the amplifier means responsive to the level of the signal therein; and means coupled to the AGC means and connected to inhibit said transfer means whenever the level of the AGC indicates substantial loss of output noise from the transducer means since the last time the counter means was reset.

9. In a system as set forth in claim 6, said system being responsive to a decrease in the frequency of the noise picked up by the transducer means as a result of an intrusion whereby there are fewer zero-axis crossings and less frequent reset outputs, and said system further comprising means responsive to a first coincidence signal to increase the rate of said fast rate pulses and to supply pulses to the short-term accumulator means at said increased rate; means responsive to a second coincidence signal at the increased rate to deliver an alarm signal; and means to disable the alarm signal delivering means and return the accumulator pulse rate to the said fast rate when no coincidence signal is detected while counting at the increased rate.

10. In a system as set forth in claim 9, memory means interposed between said long-term counter means and said comparing means, transfer means for setting into the memory means a new count from the counter means just before the latter is reset, and means for inhibiting said transfer means from updating the count in the memory means when an alarm signal has been delivered from the comparing means since the last time the counter means was reset.

11. In a system as set forth in claim 10, amplifier means coupled to the transducer means, AGC means in the amplifier means responsive to the level of the signal therein; and means coupled to the AGC means and connected to inhibit said transfer means whenever the level of the AGC indicates substantial loss of output noise from the transducer means since the last time the counter means was reset.

12. In a system as set forth in claim 9, alarm means for warning of intrusions; means for counting successive coincidence signals in an effort to reach a predetermined number thereof and connected to actuate said alarm means upon reaching that number; and means for resetting said alarm signal counting means when the accumulator means is reset before its count reaches coincidence with the memory means count determined by the comparing means.

13. In a system as set forth in claim 6, said transducer and converting means comprising a geophone connected to drive a signal-squaring amplifier.

14. An intrusion detector system, comprising:

a. transducer means for detecting noise including ambient noise and intrusioncreated noise where the frequency of the latter is lower than the former so that the number of zero-axis crossings of the transducer output decreases during an intrusion;

. means for counting pulses at a first slower rate into a counter and for resetting the counter at infrequent intervals whenever a certain number of zero-axis crossings have occurred, thereby to represent as a number in the counter the average frequency of noise as averaged over said long-term interval;

. means for counting pulses .at a second faster rate into an accumulator and for resetting the accumulator at more frequent intervals whenever a pro-- portionately smaller number of zero-axis crossings recently detected noise; and

. means for comparing the short-term count in the accumulator with the long-term count in the counter to detect a change which lowers the rate of zero-axis crossings of the noise frequency and indicates an intrusion.

15. In an intrusion detector as set forth in claim 14, a memory means interposed between the counter and the comparing means; means for updating the count in the memory means by setting into it a new count from the counter just before the latter is reset; and means for controlling the updating means to set a new count into the memory means only if no intrusion was indicated during the counting up of the counter during the past interval. 

1. A system for detecting an intrusion into a surveillance area resulting in a change in the noise characteristic within the area from ambient type noise to a composite of ambient type noise and cultural type noise, causing a substantial shift in the noise frequency spectrum, said system comprising: a. transducer means for picking up vibrations caused by ambient and cultural noise events in the area; b. means coupled to receive output from the transducer means for converting said picked up vibrations into input signals representative of the momentary dominant frequencies of the noise events; c. long-term memory means coupled to said converting means and operative to store and average over a relatively longer period of time a level representative of the average frequency of said input signals over that period; d. short-term memory means coupled to said converting means and operative to store and average over a relatively shorter period of time a level representative of the average frequency of the most recent of said input signals; e. means for indicating an intrusion; f. means connected between said memory means and said indicating means for comparing said long-term and said short-term memory levels, and responsive to a predetermined criterion of comparison and to the results of said comparison to actuate said intrusion indicating means; g. AGC amplifier means connecting said transducer means to said converting means and operative to develop an AGC level proportional to the level of the noise events; and h. means responsive to said AGC level to block actuation of said intrusion indicating means upon substantial failure of the input noise from said transducer means.
 2. In a system as set forth in claim 1, said converting means comprising means for converting the noise events into a voltage signal whose amplitude varies as the frequency thereof; and said memory means each comprising an averaging voltage-storage circuit each connected to receive said voltage signal and respectively having longer and shorter time constants, and said comparing means comprising a voltage comparison circuit connected to said memory means and operative to compare the voltage levels stored therein, and said comparison circuit being operative as long as said AGC level shows no failure of input noise to deliver an actuAting signal to said intrusion indicating means when the ratio of said levels changes beyond a preset criterion.
 3. In a system as set forth in claim 2, means responsive to the AGC level in said amplifier means and operative upon substantial failure of input noise from the transducer means to block input of said voltage signal into said long-term memory means.
 4. In a system as set forth in claim 3, said means for blocking said voltage signal further including means responsive to the delivery of an actuating signal to said intrusion indicating means for blocking said voltage signal to said long-term memory means.
 5. A system for detecting an intrusion into a surveillance area resulting in a change in the noise characteristic within the area from ambient type noise to a composite of ambient type noise and cultural type noise, causing a substantial shift in the noise frequency spectrum, said system comprising: a. transducer means for picking up vibrations caused by ambient and cultural noise events in the area; b. means coupled to the transducer means for converting said picked up vibrations into input signals representative of their momentary dominant frequencies, said converting means converting the noise events into pulse signals; c. long-term memory means coupled to said converting means and operative to store and average over a relatively longer period of time a level representative of the average frequency of said pulse signals over that period; d. short-term memory means coupled to said converting means and operative to store and average over a relatively shorter period of time a level representative of the average frequency of the most recent of said pulse signals, said memory means each comprising a counter operative to accumulate a digital count level; e. a source of clock pulses scaled to count pulses at a higher rate into the short-term memory counter and at a relatively lower rate into the long-term memory counter; f. means responsive to said pulse signals for scaling them according to the same ratio as said lower and higher pulse rates to provide two series of reset pulses, the less frequent reset pulses being connected to reset said long-term memory counter and the more frequent reset pulses being connected to reset said short-term memory counter; g. means for indicating an intrusion; h. means connected between said memory means and said indicating means for comparing said long-term and said short-term memory levels, said comparing means comprising means for detecting coincidence of the digital count levels in the counters and responsive thereto to deliver an actuating signal to said intrusion indicating means.
 6. A digital system for detecting an intrusion into a surveillance area resulting in a change in the noise characteristics within the area from ambient type noise to a composite of ambient type noise and cultural type noise causing a substantial shift in the noise frequency spectrum, said system comprising: a. transducer means for picking up the ambient and cultural noise frequencies; b. a source of pulses; c. a long-term counter means, a short-term accumulator means, and pulse rate scaler means operative to supply pulses to the long-term counter means at a first slower rate and pulses to the short-term accumulator at a second faster rate; d. means for converting the output of said transducer means into impulses representing zero-axis crossings and for scaling said crossing impulses to provide two different trains of reset outputs whose relative recurrence rates are in the same ratio as said faster and slower pulse rates; e. means for periodically resetting said long-term counter means using the less frequent reset outputs and for resetting said short-term accumulator means using the more frequent reset outputs; f. means for comparing the counts in said counter means and said accumulator means and for delivering a coincidence signal indicating an intrusion when the counts reach coincideNce.
 7. In a system as set forth in claim 6, memory means interposed between said long-term counter means and said comparing means, transfer means for setting into the memory means a new count from the counter means just before the latter is reset, and means for inhibiting said transfer means from updating the count in the memory means when an intrusion detection signal has been delivered from the comparing means since the last time the counter means was reset.
 8. In a system as set forth in claim 7, amplifier means coupled to the transducer means, AGC means in the amplifier means responsive to the level of the signal therein; and means coupled to the AGC means and connected to inhibit said transfer means whenever the level of the AGC indicates substantial loss of output noise from the transducer means since the last time the counter means was reset.
 9. In a system as set forth in claim 6, said system being responsive to a decrease in the frequency of the noise picked up by the transducer means as a result of an intrusion whereby there are fewer zero-axis crossings and less frequent reset outputs, and said system further comprising means responsive to a first coincidence signal to increase the rate of said fast rate pulses and to supply pulses to the short-term accumulator means at said increased rate; means responsive to a second coincidence signal at the increased rate to deliver an alarm signal; and means to disable the alarm signal delivering means and return the accumulator pulse rate to the said fast rate when no coincidence signal is detected while counting at the increased rate.
 10. In a system as set forth in claim 9, memory means interposed between said long-term counter means and said comparing means, transfer means for setting into the memory means a new count from the counter means just before the latter is reset, and means for inhibiting said transfer means from updating the count in the memory means when an alarm signal has been delivered from the comparing means since the last time the counter means was reset.
 11. In a system as set forth in claim 10, amplifier means coupled to the transducer means, AGC means in the amplifier means responsive to the level of the signal therein; and means coupled to the AGC means and connected to inhibit said transfer means whenever the level of the AGC indicates substantial loss of output noise from the transducer means since the last time the counter means was reset.
 12. In a system as set forth in claim 9, alarm means for warning of intrusions; means for counting successive coincidence signals in an effort to reach a predetermined number thereof and connected to actuate said alarm means upon reaching that number; and means for resetting said alarm signal counting means when the accumulator means is reset before its count reaches coincidence with the memory means count as determined by the comparing means.
 13. In a system as set forth in claim 6, said transducer and converting means comprising a geophone connected to drive a signal-squaring amplifier.
 14. An intrusion detector system, comprising: a. transducer means for detecting noise including ambient noise and intrusion-created noise where the frequency of the latter is lower than the former so that the number of zero-axis crossings of the transducer output decreases during an intrusion; b. means for counting pulses at a first slower rate into a counter and for resetting the counter at infrequent intervals whenever a certain number of zero-axis crossings have occurred, thereby to represent as a number in the counter the average frequency of noise as averaged over said long-term interval; c. means for counting pulses at a second faster rate into an accumulator and for resetting the accumulator at more frequent intervals whenever a proportionately smaller number of zero-axis crossings have occurred, thereby to represent as a number in the accumulator the short-term frequency of recently detected noise; and d. meAns for comparing the short-term count in the accumulator with the long-term count in the counter to detect a change which lowers the rate of zero-axis crossings of the noise frequency and indicates an intrusion.
 15. In an intrusion detector as set forth in claim 14, a memory means interposed between the counter and the comparing means; means for updating the count in the memory means by setting into it a new count from the counter just before the latter is reset; and means for controlling the updating means to set a new count into the memory means only if no intrusion was indicated during the counting up of the counter during the past interval. 